Many types of bicycle trailers are known in prior art. As can be easily understood, it may be greatly advantageous to be able to store or carry a bicycle trailer into a compact form. Already known in prior art, there are certain foldable or collapsible bicycle trailers. Examples of such bicycle trailers are given in U.S. Pat. No. 5,020,814 and U.S. Pat. No. 5,474,316. U.S. Pat. No. 5,020,814 discloses a collapsible trailer including a rigid chassis frame, and hinged side rails rotatable on corresponding hinge members between an extended vertical position and a folded position essentially parallel to the chassis frame. A side rail locking bar is provided to lock the side rail in the extended position. US patent no. 5,474,316 discloses a folding trailer comprising an upper frame member, a lower frame member and a pair of folding units. The weakness of folding locations is often a source of problems, and developments in stronger and simpler mechanisms for compacting a bicycle trailer are needed.